This disclosure relates to seismic exploration for oil and gas and, in particular but not by way of limitation, to marine seismic surveying using marine vibrators.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey may involve deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations, creating pressure changes and vibrations along the way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), and others are sensitive to particle motion (e.g., geophones); industrial surveys may deploy one type of sensor or both types. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. However, “marine” surveys may not only be conducted in saltwater environments, but also in fresh and brackish waters. In one type of marine survey, called a “towed-array” survey, an array of seismic sensor-containing streamers and sources is towed behind a survey vessel. In one type of survey, called a “marine vertical seismic profile (marine VSP)” survey, an array of sensors is deployed in a borehole and the seismic source is either moving (e.g. towed behind a vessel) or stationary (e.g. suspended from a structure such has a drilling rig). In one type of marine survey, incorporating both hydrophones and geophones, called an “Ocean Bottom Cable (OBC)” survey, sensors are laid on the seabed. In another type of marine survey, the sensors are deployed in other ways and the seismic source is deployed in the water in some way, where the sensors or sources can be either moving or stationary. Other surveys are known as “land” surveys because they are conducted on land environments. Land surveys may use dynamite or seismic vibrators as sources. Arrays of seismic sensor-containing cables are laid on the ground to receive seismic signals. The seismic signals may be converted, digitized, stored or transmitted by sensors to data storage and/or processing facilities nearby, e.g. a recording truck. Land surveys may also use wireless receivers to avoid the limitations of cables. Seismic surveys may be conducted in areas between land and sea, which is referred to as the “transition zone”.
Theoretically, in marine seismic surveys, the sources can be impulsive sources (e.g. airguns) or continuous sources (e.g. marine seismic vibrators). However, in general, marine seismic vibrators are not used in practice. It is desirable to be able to make marine seismic vibrators another practical type of source for marine seismic surveys.